Making steel without coal

Can the world produce steel without using coal?

The short answer to this question is no, not at scale, at the present time.

The issue

Steel is an alloy of iron with carbon (0.002% - 2.1% by weight), and with other metals as needed. Typical additives are: nickel, chromium, manganese, molybdenum, titanium, vanadium, or tungsten, depending on physical properties sought, e.g., anti-rust, light, tough, heat resistant, elastic, or cheap. As a material, steel combines high tensile strength with low cost. It is one of the building blocks of civilisation.

For an idea of scale, the world produces around 1.6 billion tonnes of steel every year, half of this in China. Other major producers are: Japan, India, the US, Korea, and Russia. This is an energy- intensive business, in which technology advances have reduced the energy requirement by 60% over the last 50 years.

Today, around 0.8 tonne of coal is used and 2.1 tonnes of CO2 emitted from the raw materials to produce every tonne of steel (separate from any coal or gas used to generate the electricity also required). The International Energy Agency estimates that the global iron and steel industry accounts for 6.7% of world CO2 emissions. This is a significant figure, and will need to improve as the world transitions to a low-carbon future. This will be challenging.

Making steel using traditional methods

First, iron is smelted from its mineral ore. This is usually an iron oxide such as haematite or magnetite. A furnace temperature exceeding 1600C will release the iron, in the form of "pig iron", so called for the shape of the ingots. This is a brittle material containing as much as 4.5% carbon. Historically the first fuels used to heat the smelters were wood and, later, charcoal (which is made from wood).

The earliest steels appeared in Anatolia (from 1800 BCE), East Africa (from 1400 BCE), South India (from 600 BCE), and in China (from 400 BCE). The Roman military used steel weapons. The production of steel from pig iron requires a reduction in carbon content, to produce a useful metal.

The change to using coal in steel-making dates from the 11th Century in the Yellow River region of China, where trees were sparse. Specifically, the coal was converted into "coke" by heating it in oxygen-starved conditions to drive off embodied water and volatile organic chemicals. This produces a hard, grey, porous material composed mainly of carbon. This has a much higher energy value than coal, and is better geared to producing high temperatures for smelting.

Coke came into use in Great Britain in the 1700s, partly because of its superior crushing strength to that of coal. Blast furnaces for making iron and steel could be built taller and bigger, to improve economies of scale. The growing demand for steel as the Industrial Revolution progressed far exceeded the ability of forests to provide the fuel and source of carbon.

Now, nearly all new steel globally is produced using iron oxide and coking coal (aside from the recycled steel discussed below). Coking coal is usually bituminous-rank coal with special qualities that are needed in the blast furnace.

Making steel in New Zealand

New Zealand Steel uses a titano-magnetite ironsand at their Glenbrook plant, and exports the same ironsand to be used as a minor contribution in conventional steel plants. The plant uses a direct reduction process to make iron from the ironsand before this is turned into steel. No other operation in the world makes steel in the same way. Major improvements have been made in energy efficiency through co-generation (using waste heat) where New Zealand Steel produces up to 70% of its own electricity requirements.

Making steel without coal

This is a holy grail for emissions chasers and there has been considerable international research on ways of reducing or eliminating CO2 emissions.

Recycling steel

Around 500 million tonnes of steel is recycled every year from scrap, or 31% of total global steel production. This is a very high percentage of recycling for any material. When claims are made that steel can be made in electric-arc furnaces (instead of emissions-intensive blast furnaces), this is what is being talked about.

To highlight the importance of recycling in steel production, the World Steel Association says that the average blast furnace needs 800kg of coal to produce a tonne of steel while the average electric arc furnace (using mainly recycled steel) needs just 16kg of coal.

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Of course, to manage CO2 emissions from steel recycling, the electricity input would have to be renewable.

In general, recycling is done as economics, or regulations or conscience dictates. As to claims that 80% of steel could be recycled, this will depend on the sector. NZ Steel has estimated that for buildings, the level of steel recycling could reach as high as 85% 1.

Use bio-carbon in steel making

Bio-carbon is made from wood, or wood waste. Importantly, this source of carbon for steel-making can only qualify as renewable if wood is being created more quickly (by growing trees) than it is being chopped down and burned.

To the extent that charcoal could be used in steel-making (or cement-making) around the world to replace coal, it is questionable whether this use of wood would be considered environmentally sustainable, especially if this huge shift in land use displaced food production.

In New Zealand, NZ Steel 2 has trialled 9000 tonnes of bio-carbon supplied by Carbonscape, as a method of making low-emissions steel. To put this into context, NZ Steel uses around 800,000 tonnes of New Zealand or imported coal each year to make iron and then steel at its Glenbrook mill. As yet, the jury is out on the success of this trial as a commercially-viable method.

Smelt iron using electrolysis

An intriguing way of separating iron from its ore at MIT was reported in Scientific American (May 2013) There was a flurry of media interest at the time but a promised commercial-scale demonstration is yet to appear.

The method is to use a receiving environment of molten metal oxides, in which the iron ore would dissolve, and then pass an electric current through it, to precipitate the iron out onto positively- charged electrodes. To date very expensive platinum or iridium has been used as an electrode, because these metals can withstand 1600C. The breakthrough has been to create much cheaper chromium alloys that can also do the job. A 30% increase in energy efficiency is also claimed.

So while research continues in New Zealand and around the globe, there is currently no viable alternative to using coal in large-scale production of steel.

References

  1. http://www.nzsteel.co.nz/sustainability/recycling/
  2. http://www.nzsteel.co.nz/
  3. http://www.scientificamerican.com/article/cleaner-cheaper-way-to-make-steel-uses-electricity/